Rinnai CAH Series Instalation And Operating Manual page 14

Domestic Priority Switch Part #
REU-OPU3
Normally Closed (NC) switch that connects to the PC Board in the
Rinnai tankless water heater or boiler.
Allows the tankless water heater or boiler to give priority to domestic
hot water by shutting off the air handler when necessary. When used
with a hydronic air handler, the switch gives priority to domestic hot
water. When domestic hot water demand exceeds a certain point, the
air handler will turn off to ensure the demand is met.
Product images are for illustrative purposes only.
14 Canadian Air Handler Installation and Operation